This invention relates to the casting of steel strip. It has particular application for continuous casting of thin steel strip less than 5 mm in thickness in a roll caster.
In a roll caster, molten metal is cooled on casting surfaces of at least one casting roll and formed in to thin cast strip. In roll casting with a twin roll caster, molten metal is introduced between a pair of counter rotated casting rolls that are cooled. Steel shells solidify on the moving casting surfaces and are brought together at a nip between the casting rolls to produce a solidified sheet product delivered downwardly from the nip. The term “nip” is used herein to refer to the general region in which the casting rolls are closest together. In any case, the molten metal is usually poured from a ladle into a smaller vessel, from where it flow through a metal delivery system to distributive nozzles located generally above the casting surfaces of the casting rolls. In twin roll casting, the molten metal is delivered between the casting rolls to form a casting pool of molten metal supported on the casting surfaces of the rolls adjacent to the nip and extending along the length of the nip. Such casting pool is usually confined between side plates or dams held in sliding engagement adjacent to ends of the casting rolls, so as to dam the two ends of the casting pool.
When casting thin steel strip with a twin roll caster, the molten metal in the casting pool will generally be at a temperature of the order of 1500° C. and above. It is therefore necessary to achieve very high cooling rates over the casting surfaces of the casting rolls. A high heat flux and extensive nucleation on initial solidification of the metal shells on the casting surfaces is needed to form the steel strip. U.S. Pat. No. 5,760,336 incorporated herein by reference describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial portion of the metal oxides formed are liquid at the initial solidification temperature, and in turn, a substantially liquid layer formed at the interface between the molten metal and each casting surface. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, the disclosures of which are incorporated herein by reference, nucleation of the steel on initial solidification can be influenced by the texture of the casting surface. In particular, International Application AU 99/00641 discloses that a random texture of peaks and troughs in the casting surfaces can enhance initial solidification by providing substantial nucleation sites distributed over the casting surfaces.
Attention has been given in the past to the steel chemistry of the melt, particularly in the ladle metallurgy furnace before thin strip casting. We have given attention in the past to the oxide inclusions and the oxygen levels in the steel metal and their impact on the quality of the steel strip produced. We have now found that the quality of the steel strip and the production of the thin steel strip is also enhanced by control of the hydrogen levels and nitrogen levels in the molten steel. Controlling hydrogen and nitrogen levels has in the past been the subject of investigation in slab casting, but to our knowledge has not been a focus of attention in thin strip casting. For example, see Control of Heat Removal in the Continuous Casting Mould, by P. Zasowski and D. Sosinsky, 1990 Steelmaking Conference Proceedings, 253–259; and Determination and Prediction of Water Vapor Solubilities in CaO—MgO—SiO2 Slags, by D. Sosinsky, M. Maeda and A. Mclean, Metallurgical Transactions, vol. 16b, 61–66 (March 1985).
Specifically we have found that by controlling the hydrogen and nitrogen levels in the steel melt, with low levels of sulfur in the steel, plain carbon steel strip having unique composition and production qualities can be produced by roll casting. There is provided a method of casting steel strip comprising:
introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a free nitrogen content below about 120 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
forming a casting pool of molten metal on the eastine surfaces of the casting rolls; and
solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
The method of casting steel strip may be carried out by the steps comprising the following:
assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;
introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on casting surfaces of the casting rolls with the end closures confining the pool, with the molten steel having a free nitrogen content below about 120 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres; and
counter-rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to provide for the formation of thin steel strip; and
forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
Alternatively, there is provided a method of casting steel strip comprising:
introducing molten plain carbon steel on casting roll surfaces of at least one casting roll having a free nitrogen content below about 100 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
forming a casting pool of molten metal on the casting surfaces of the casting rolls; and
solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
The method of casting steel strip may be carried out by the steps comprising the following:
assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;
introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on casting surfaces of the casting rolls with the end closures confining the pool, with the molten steel having a free nitrogen content below about 100 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
counter-rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to provide for the formation of thin steel strip; and
forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
As a further alternative, there is provided a method of casting steel strip comprising:
introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a free nitrogen content below about 85 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
forming a casting pool of molten metal on the casting surfaces of the casting rolls; and
solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
The method of casting steel strip may be carried out by the steps comprising the following:
assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;
introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting surfaces of the casting rolls with the end closure confining the pool, with the molten steel having a free nitrogen content below about 85 ppm and a free hydrogen content below about 6.9 ppm measured at atmospheric pressure and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;
counter-rotating the casting rolls and solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to provide for the formation of thin steel strip; and
forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip. The content of the free hydrogen may be below about 6.5 ppm, and sum of partial pressure of nitrogen and partial pressure of hydrogen in the introduced molten metal may be no more than 1.0 atmosphere.
In any of these methods, the free nitrogen content may be 60 ppm or less, and the free hydrogen content may be 1.0 to 6.5 ppm. The free hydrogen content may, for example, be between 2.0 and 6.5 ppm or between 3.0 and 6.5 ppm.
Plain carbon steel for purpose of the present invention is defined as less than 0.65% carbon, less than 2.5% silicon, less than 0.5% chromium, less than 2.0% manganese, less than 0.5% nickel, less than 0.25% molybdenum and less than 1.0% aluminum, together with of other elements such as sulfur, oxygen and phosphorus which normally occur in making carbon steel by electric arc furnace. Low carbon steel may be used in these methods having a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.01% to 2.0% by weight, and a silicon content in the range 0.01% to 2.5% by weight, and low carbon cast strip may be made by the method. The steel may have an aluminum content of the order of 0.01% or less by weight. The aluminum may, for example, be as little as 0.008% or less by weight. The molten steel may be a silicon/manganese killed steel.
In these methods, the sulfur content of the steel may be 0.01% or less; and the sulfur content of the steel may be 0.007% by weight.
In these methods, the free nitrogen may be measured by optical emission spectrometry, calibrated against the thermal conductivity method a described below. The free hydrogen levels may be determined by a Hydrogen Direct Reading Immersed System (“Hydris”) unit, made by Hereaus Electronite.
The maximum allowable free nitrogen and free hydrogen levels may be for total pressure not to exceed 1.0 atmospheres. Higher pressures may be utilized in certain conditions, and the levels of free nitrogen and free hydrogen can be corresponding higher. For example, as explained below, a ferrostatic head may be 1.15, causing the free nitrogen levels and free hydrogen levels to be higher as shown in FIG. 3. But for purposes of the parameters of the present methods, the free nitrogen and free hydrogen levels are measured a 1.0 atmospheres even through the actual levels of free nitrogen and free hydrogen in the molten metal are higher when the methods are practiced with higher positive atmospheric pressure.
The present invention provides cast steel strip with unique properties that are described by the methods by which it is made. This steel strip is plain carbon steel.